The Pittsburgh, United Statesbased company, founded in 2013, has notched up nearly 28 million users worldwide with the most popular languages on offer being English and Spanish.

The app, which makes the lengthy process of learning languages into a fun-filled game, is already being used by over 1.5 lakh Indians who are learning languages as varied as Spanish, French and German through it.

"It took this long because we were not ready for non-Roman script. But now we are," said Luis von Ahn, chief executive of Duolingo who is also an associate professor in the Computer Science Department at Carnegie Mellon University in Pittsburgh.

Also being launched are lessons in other non-Roman scripts such as Chinese and Japanese.

The company does not hire translators or experts to deliver lessons on the application. The incubator is completely crowd sourced, which explains the requests to learn meme-based internet languages such as Lolcat and Pirate English The language incubator, as Duolingo calls it, engages with interested volunteers who would want to start new courses and gets them to moderate the learning content.

"We will always be free and the business will continue to be sustainable," said von Ahn, 34, who was born in Guatemala, and sold his earlier venture reCAPTCHA to Google in 2009. reCAPTCHA is a Web service that enables a squiggly set of characters to protect websites from bots and enables digitisation of books.

Duolingo, whose largest markets are Europe and Latin America, also does not carry advertisements.

Instead, what von Ahn and his team have devised is a monetisation model that involves learners translating real-world documents as they learn the language on the app.

Last October, the company partnered with CNN and website BuzzFeed to use its translations. In May, the one-year-old app will begin to offer certification for English language proficiency beginning with a platform for English language testing for Android phone users. The service will be available for other languages from July onwards.

"It will not be $200 test but around a $20 test that you will be able to take from your home," said von Ahn, as opposed to standard tests like TOEFL or IELTS which require you to travel to designated test centres for certification.

Nearly 85 % of users are learning languages on Duolingo using their mobile phones, logging in for 10-minute bursts of lesson while riding a train or waiting in a queue.

Education technology entrepreneur K Ganesh, is of the view that bite-size learning as this trend is called, is here to stay. "This generation has a short attention span and people want to learn in bite-sizes which mobiles enable," said the 50 year-old who sold online tutoring firm Tutor Vista to UL' Pearson for over `1,000 crore in 2011. "That's why Duolingo as a company will succeed," he said The US-company which was valued at "hundreds of millions of dollars" at its most recent fund raising in February, received $ 20 million of equity capital led by Silicon Valley firm Kleiner Perkins Caufield & Byers.

"We have enough money to last us for several years without any revenue. So our focus really right now is to become the standard way to learn languages," said von Ahn..